Many processes are in use where heat transfer to and from particulate solids is an important unit operation. With the resurgence of the synthetic fuels industry and increasing usage of calcined lime for SO.sub.2 control, this unit operation will become even more important. However, high fuel costs and conservation efforts dictate that these processes achieve a high degree of thermal efficiency.
Generally speaking, in order to achieve efficient heat exchange, countercurrent operation or highly staged operation is required. The latter option can be quite expensive; thus the number of stages is usually limited to two or three and heat efficiency is sacrificed. Countercurrent operation, which is used extensively for gas/liquid, gas/gas, and liquid/liquid heat exchange, is both efficient and relatively inexpensive. Nonetheless, countercurrent heat exchange between solids is rarely employed, for a variety of reasons.
U.S. Pat. No. 2,725,348 discloses a process in which solid heat carrier particles may be permitted to fall more or less freely through a fluidized mass of process solids at a velocity controlled by that of the fluidizing gases. However, the employment by the patentees of a dense fluidized bed renders the process unattractive from the standpoint of thermal efficiency. The present invention overcomes this deficiency and provides a method of heat transfer between solids which is thermally efficient.